1. Field of the Invention
This invention pertains to an apparatus and method for comminuting particles of a frangible material, and particularly for the production of filter and material, by impact milling.
2. Description of the Prior Art
Filter aids of the present type are finely divided solids of inert materials, like diatomaceous earth or expanded perlite, of such particle size and shape as to form a filter bed or cake of such porosity and permeability as to permit fairly free passage of liquids without allowing the passage of any of the solids which are to be removed from the liquid.
The function of a filter aid of the type here involved is to maintain the porosity and permeability of a filter, to increase the rate of flow, and to assist in clarifying the liquid. High flow rate through the filter coupled with high clarity of the filtrate requires close control over size and shape of the filter aid particle.
There are several known types of apparatus for milling particles of a frangible material into small sizes. Among these are the hammer or ball mill, and the impact mill, including the jet and anvil mill. Each of these mills has certain advantages and disadvantages for grinding particular types of materials to different particle size ranges. For example, impact milling, where the coarse particles are accelerated to a high velocity and allowed to impact against each other or against an anvil-like memeber, is advantageously used in the low range for dry grinding, as compared to a ball mill. In the case of a ball mill, the material and balls are surrounded by an air film when the material approaches the extremely fine, dry state. Material is squeezed out from between the balls and therefore eludes further grinding. Stated otherwise, the air film seems to work as a cushion to protect the material from further comminution.
The U.S. Pat. No. 2,798,674 to Denning discloses a method for filter aid production and shows an impact milling apparatus wherein the particles are accelerated by a rotating impeller and allowed to impact against a stationary plate. In the Denning process, the impacted material is collected and screened, and the coarse particles are returned to the apparatus for further impacting until substantially all of the particles are less than a predetermined size.
While the apparatus shown in Denning is suitable for comminuting particles of frangible materials as well as filter aid preparation, high-speed rotating equipment, in general, has a high initial cost due in part to the manufacturing tolerances required to insure vibration-free operation. The abrasive nature of some of the materials comminuted requires that the bearings of the rotating parts have special shielding to prevent premature bearing failure, which shielding can increase the complexity of the design and, consequently, the cost. Also, some impeller bearing mounting configurations used in the rotary impact millers of the type shown by Denning require periodic shut-downs for maintenance which can contribute substantially to the overall production costs.
Another type of impact milling apparatus, and one which does not utilize expensive rotary equipment to accelerate the particles, is the jet and anvil mill. In the jet and anvil mills, the particles of material to be comminuted are aspirated into a linear jet, accelerated by the action of the jet to a high velocity and thereafter are projected against an anvil causing fragmentation of the particles. Examples of apparatus utilizing the jet and anvil principle are found in U.S. Pat. Nos. 3,876,156 and 3,688,991.
The U.S. Pat. No. 3,876,156 to Muschelknautz et al., discloses apparatus for accelerating particles of a frangible solid in a linear jet-tube and then impacting them either against a stationary anvil or against similarly accelerated particles traveling in the opposite direction. A diffusor section is provided to create a rarified wherein the pressure head build-up on the anvil is minimized and whereby fewer particles will be deflected by the pressure head and carried past the anvil by the jet stream without impacting and without being comminuted. The relatively high jet speeds employed in comminuting apparatus of the type shown in Muschelknautz et al. in an effort to overcome the effects of the pressure head usually are reflected in higher costs for equipment and for power expenditures. The device shown in FIG. 1 of the Muschelknautz et al. reference is designed to operate in the sonic range, that is, with gas speeds &gt; Mach 1. Friction losses tend to increase dramatically for speeds &gt; Mach 0.3.
The U.S. Pat. No. 3,688,991 to Andrews discloses apparatus for impinging particles accelerated by a jet against a plurality of rotating anvils which are alternately introduced into, and then removed from, the jet. The rotating anvil apparatus of Andrews is intended to minimize the adverse pressure head buildup on each individual anvil and thus serve to increase the momentum of the particles at impact and decrease the number of particles deflected around the anvil by the diverging jet stream. Again, as in the apparatus shown in Denning, the use of rotating equipment can result in increased costs.
It is clear from the foregoing that those skilled in the art of comminuting frangible particles, using apparatus incorporating the jet and anvil principle, have gone to considerable lengths to minimize or eliminate the effect of the adverse pressure heads built up on stationary anvils. Therefore, a method and apparatus of comminuting which would not only tolerate but actually utilize this hydraulic phenomenon could be expected to result in substantial cost savings due to a decrease in the complexity of the comminuting apparatus and a reduction in the required power input.